The production of polymer film, for instance polyethylene terephthalate film, involves a sequence of process steps which includes the extrusion of the molten resin in the form of a film, quenching the extruded film, and thereafter sequentially stretching the film in both the longitudinal and transverse directions. For this purpose, a polymer film orientation line generally will comprise an extruder, casting rolls and forward draw rolls followed by textile-type tenter apparatus in which transverse stretching at elevated temperature occurs. A feature of presentday lines is that they operate at very high speeds, frequently by way of example, in excess of 200 meters per minute.
A critical step in polymer film production is proper orientation of the cast and drawn film in the tenter apparatus, and in particular proper introduction of the cast and drawn film into the tenter clips. In operation, it is impractical to stop the line to insert the film into the tenter clips. It is possible to reduce line speed, but this reduction is limited by equipment and process reasons such that in high speed lines the minimum speed available for threading film into the tenter clips is still dangerously high.
It is known to provide an accumulator system to substantially slow down the lineal speed of the film so that it may be safely and accurately positioned in the tenter apparatus clips, but this requires the use of complicated and unduly expensive equipment.
Another problem unique to polymer films, not experienced with conventional webs, is the difficulty in engaging the polymer film with a feed device. It is undesirable to puncture the film with conventional threading devices, and polymer films have slippery or relatively frictionless surfaces which make engagement with other devices relatively difficult.